3G4 PROCEEDINGS OF THE AMERICAN ACADEMY. 



slipping on the rod. The change of length of the cylinder itself is 

 measured directly from the outside. 



In the previous work the rod was enclosed in a cylinder of soft tool 

 steel. The maximum pressure, 6500 kgm., was beyond the elastic 

 limit of this steel, so that the material did not remain isotropic. That 

 the elastic limit was exceeded is shown by the considerable hysteresis 

 in the observed elongation of the cylinder. The fact that there is 

 hysteresis suggests that possibly there may be also some slight degree 

 of warping. It was stated in the former paper that the only assump- 

 tion made in the whole work was in regard to this matter of warping, 

 that is, whether the change of length of the cylinder is the same in- 

 ternally as externally. The effect is probably slight, and in the absence 

 of any plausible way of calculating it was neglected altogether. 



Apart from the question of the method used in the present work, 

 and not affecting its validity in the least, there was an error made in 

 the previous paper in the calculations from the data. Correction for 

 this error increases the former value for the compressibility of this steel 

 from 5.30 X 10"' to 5.59 X 10"'' at 20°. This also will change the 

 value for the compressibility of mercury, as will be explained later. 



The tool steel cylinder was not available for the present work, be- 

 cause it would not reach the pressure reached here, 10,000 kgm./cm.'^ 

 Another cjdinder of nickel steel of the same dimensions as the former 

 one of tool steel was made, therefore, and hardened in oil. With this 

 heat treatment, the elastic limit should be somewhere near 15,000 kgm. 

 The pressure was purposely kept at a low value, never exceeding 10,000 

 kgm., so as to run no risk of producing set in the inner layers of the 

 cylinder. That no such heterogeneity was introduced is made probable 

 by the fact that the cylinder showed no hysteresis, the relation between 

 extension and pressure being linear both for increasing and decreasing 

 pressure. The former measurements of this extension were made di- 

 rectly with a microscope, reading to 0.001 mm. For this work here, a 

 much more sensitive scheme was used, an adaptation of the Maarten's 

 mirror device so fast coming into general use among engineers. The 

 magnification employed here gave a motion of the scale of 5 cm. for an 

 actual elongation of 0.025 mm. The sensitiveness is, therefore, at least 

 twice that of an interference system, there is also the advantage of 

 great simplicity and steadiness, and best of all, the reading is given 

 directly on a scale, there being no necessity for keeping track of fringes. 

 The Maarten's mirror device disclosed no hysteresis in the elongation 

 of the nickel steel cylinder. The freedom from heterogeneity as shown 

 by the absence of hysteresis makes plausible also the absence of warp- 

 ing, although no direct measurements of this warping were possible. 



